The goal of this study is to determine if systemic vasocilation and/or pulmonary vasoconstriction can reduce left-to-right shunting and favorably alter the abnormal hemodynamics which cause congestive heart failure in the presence of communications between the systemic and pulmonary circulations. Young lambs in which large ventricular septal defects had been previously produced will be the subjects under study. Hemodynamic data will be collected using techniques which are used during clinical cardiac catheterization. The lambs will be randomly divided into four groups with Group 1, the controls, receiving no drugs. Group II animals will receive the alpha adrenergic receptor blocker Phenoxybenzamine and Group III the pulmonary vasoconstrictor promethazine. Group IV will be given both of these agents. Pressures, flows, resistances and left-to-right shunting will be evaluated following both acute and chronic administration of these drugs and the experimental groups compared to the control group and to each other. Following chronic drug administration, the animals will be sacrificed and the lung tissues examined microscopically to compare different treatment effects on the development of pulmonary vascular occlusive disease.